FR2596775A1 - Hard multilayer coating produced by ion deposition of titanium nitride, titanium carbonitride and i-carbon - Google Patents

Abstract

Hard multilayer coating produced by ion deposition. According to the invention the hard coating comprises firstly a layer of titanium nitride, then a layer of titanium carbonitride, then a layer of i-carbon. According to the invention the deposition of the various layers is performed in an ion deposition device under low pressure comprising a substrate-carrier cathode 8, a device for evaporating titanium 3 using an electron gun 5, gas delivery piping 10, 11, a gas ionisation filament 14 and a positively polarised third electrode 9. The deposition of titanium nitride is obtained by discharge in nitrogen and simultaneous evaporation of titanium. The progressive replacement of nitrogen by a hydrocarbon makes it possible to obtain the carbonitride deposit. The finish layer of i-carbon is deposited after the removal of the nitrogen and of the evaporation of titanium.

Description

Hard multilayer coating produced by ionic nitride deposition of
titanium, titanium carbonitride and i-carbon.

 The present invention relates to a multilayer hard coating produced by ion deposition and comprising a layer of titanium nitride, and also to a method for producing this coating and to a device for carrying out this method.

 We know that by combining several layers of different materials, we can obtain a composite coating whose properties are significantly improved compared to the use of only one of these materials.

 Coatings made of hard materials, such as carbide, or titanium nitride are well known and widely used in the industry to obtain either increased surface hardnesses, an improvement in the coefficient of friction, better protection against corrosion, and often a combination of these favorable effects.

 The methods for producing these coatings are also well known and very diverse, ranging from chemical deposition methods to ion deposition methods, the latter generally consisting in projecting ions of the different materials intended to constitute the coating on the surface of the body or substrate to be covered. The processes for the ionic deposition of titanium carbide and titanium nitride are already used on a large scale in industry, for example for coating the cutting edge of cutting tools.

 The ionic deposition methods are themselves diverse in terms of the conditions of use and the. determination of the different deposition parameters such as the intensity of the electric field, the gas pressure prevailing in the enclosure of the device, the temperature of the substrate to be covered, and also the order in which the deposits are made in multilayer coatings , as well as the nature of the materials used.

 Among the advantages of ion deposition or bombardment, mention may be made of improved adhesion of the coating to the substrate, the formation of dense and uniform layers, the activation of phase transitions or chemical reactions, without the temperature of the substrate rising. considerably, and finally the production of metastable structures, for example of atomic mixtures without stoichiometric restriction.

It was thus possible to carry out experimentally the ionic deposition of layers of carbon in metastable phase, the properties of which are similar to those of diamond, this carbon being called "i-carbon", to recall the essential role of the ions in its preparation process. The article by C. Weissmantel, "Preparation,
Structure and properties of hard coatings on the basis of iC and i-BN ", published in" Thin Films from free atoms and particles "(Klabunde, Academic Press, 1985) describes in detail the properties and processes for the preparation of i-carbon. The latter has advantageous characteristics, among others a very high hardness, a low coefficient of friction, a considerable resistance to compressive stresses, and remarkable anticorrosion properties.

 On the other hand, achieving perfect adhesion of the i-carbon to a metal substrate presents some difficulties.

 The object of the invention is to combine the advantages of the i-carbon layers with those of the well-known titanium nitride coatings, while ensuring maximum adhesion of the layers to the substrate, and to each other. In accordance with the present invention, this object is achieved by the fact that the multilayer hard coating produced by ionic deposition on the surface of a substrate and comprising a layer of titanium nitride TiN, further comprises, on said layer of TiN, d first, a transition layer of titanium carbonitride Ti (C, N), then a finishing layer of i-carbon iC.

 Thanks to the invention, a coating is obtained which simultaneously has a very high hardness, a low coefficient of friction and very good anticorrosion properties, and which moreover has a considerably reinforced adhesion to the substrate.

 It turned out that in this arrangement, the structure of the i-carbon ensures a good bond with the underlying layer.

 According to an advantageous embodiment of the invention, the coating further comprises a layer of titanium deposited between the substrate and the layer of TiN.

 The multilayer coating according to the invention has, by the presence of i-C in the outer layer, all the advantages of this. In particular, the hardness of the coating is typically greater than 30 G.Pa (Vickers hardness) and can reach and even exceed 50 G.Pa, while the hardness of known TiN coatings is only of the order of 25 G.Pa . Likewise, the coefficient of friction is less than 0.1, and can even reach 0.04, while the coefficient of friction of TiN is of the order of 0.2. In addition, the corrosion resistance has been significantly improved.

 The previous characteristics of the coating according to the invention make its use particularly advantageous in the field of cutting tools such as drills and cutting inserts thanks to its hardness, for mechanical parts in rapid relative movement to reduce friction and wear. The anticorrosion characteristics are advantageous for the production of hard, protective layers on various materials (metals, ceramics, glass, etc.). The coating can also advantageously be used as an inert layer on medical implants.

 In addition, depending on the thickness of the layer of iC, the color of the coating can vary from the color gold, for layers of iC of thickness less than 0.5ym, to the black color for thicknesses of iC greater than 2ym . For intermediate thicknesses, the colors are reddish or bluish. In all cases, a shiny appearance is observed. These last characteristics allow the realization of decorative effects, simultaneously with the protection and the hardness provided by the multilayer coating.

 The subject of the invention is also a process for the preparation of a multilayer hard coating by ionic deposition comprising the deposition on the surface of a substrate, of a layer of titanium nitride, characterized in that one then performs, on the iiN layer, an ionic deposition of a Ti (C, N) layer, then an iC layer.

More particularly, according to the process which is the subject of the invention, successively is carried out
- cleaning the substrate by ion bombardment obtained by discharge in an inert gas,
- deposition of the titanium layer by evaporation of the titanium while maintaining the discharge,
- the deposition of the titanium nitride layer by gradual replacement of the inert gas with nitrogen,
the deposition of the titanium carbonitride transition layer by gradual replacement of nitrogen with a hydrocarbon,
- The deposition of the i-carbon layer by stopping the evaporation of titanium and removing nitrogen, and continuing the discharge in the hydrocarbon.

 The successive preparation of the different layers by ion deposition in fact ensures atomic interpenetration at the interfaces, resulting in a gradual transition from one composition to the next, and consequently, very good adhesion of the layers to each other. The ionic deposition of Ti (or TiN) directly on the surface of the substrate provides, for its part, a "bonding layer" improving the adhesion of the coating on the.

substrate.

 The invention also relates to an ion deposition device for implementing the above method, this device comprising a sealed enclosure comprising a cover, and in which are placed: a substrate-carrying cathode surrounded by a screen, a device titanium evaporator comprising an electron gun, gas supply means, a retractable movable screen located above the titanium evaporation device, and orifices connected to a vacuum pump. , this enclosure consists of two superimposed chambers separated by an intermediate partition, each chamber having an orifice connected to a vacuum pump, the electron gun being placed at the level of the intermediate plate and the upper chamber being provided on its periphery with solenoids . A tungsten filament heated by the Joule effect is placed between the substrate-carrying cathode and the movable screen and a third positively polarized electrode is placed above said filament and under said cathode.

Other characteristics and advantages will appear in the description of a particular embodiment of the invention which will be made in relation to the appended drawings in which
- Figure 1 is a simplified representation in partially exploded view of an ion deposition device according to the invention;
- Figure 2 is a top view of an electrode of the device called the third electrode, and a filament placed under said third electrode showing their shape and relative arrangement.

 We will first describe the ion deposition device of Figure 1, then we will detail the characteristic steps of the process implemented with said device to achieve the hard multilayer coating of TiN, Ti (C, N) and i-C.

 The device of FIG. 1 is an ion deposition apparatus comprising inter alia the various organs essential in this type of apparatus and known by the devices of the prior art. In particular, the auxiliary devices which are nevertheless essential for operation, such as vacuum pumps, sources of electrical energy, sources of the various gases, etc. are not shown, as well as their command and control means.

The device of FIG. 1 consists of a sealed enclosure 1 made of stainless steel of cylindrical shape with a vertical axis. This enclosure 1 comprises an upper part 18 and a lower part 19, separated by an intermediate plate 2. The upper part 18 is closed by a cover 17 which supports. the following different organs, placed inside the upper enclosure 18
- In the center, a substrate-carrying cathode 8 of generally cylindrical shape surrounded by a screen 15 and the lower face of which, perpendicular to the axis of the device, supports the substrate which is coated. The substrate-carrying cathode 8 does not present any notable peculiarity compared to known cathodes and we will not describe it in more detail. We only recall that cathode 8 is negatively polarized with respect to the mass of enclosure 1 under a voltage of a few thousand Volts.

 - Under the substrate-carrying cathode 8, an electrode, called the third electrode 9, and under it, a tungsten filament 14 heated by the Joule effect.

 - Tubes 10, 11 for supplying the various gases, which pass through the cover in leaktight manner, are connected by their external ends to the gas sources, not shown, and which open into the space located under said filament 14 and said third electrode.

 - A retractable mobile screen 16 controlled by a rotary handle located above the cover 17, and intended to be interposed between the electron gun 5 and the substrate-carrying cathode 8, below the internal through ends of the pipes 10, 11 .

 The functions of the third electrode 9, of the filament 14.

and of the mobile screen 16, will be described in more detail below.

 The upper part 18 of the enclosure 1 also comprises a pumping orifice 7, intended to be connected to a vacuum pump, in order to obtain the vacuum required inside said upper part 18.

 Solenoids 12, 13 are arranged around the upper part 18 in windings of vertical axis, and are intended to create a magnetic field in the enclosure to concentrate the ionized plasma created in the process during the discharge induced by the polarization of the substrate-carrying cathode.

 The lower part 19 of the enclosure 1 also includes a pumping orifice 6, connected to a vacuum pump. Inside this part, and fixed by suitable means on the intermediate plate 2, is the electron gun 5 and the titanium evaporation device. These organs are of a type well known in the prior art, and no further description will be made of them. It is however recalled that these organs are intended in the ion deposition process, to vaporize the titanium contained in the device 3 by electronic bombardment.

 The plate 2 is pierced with an orifice closable by a valve 4, and capable of bringing the upper 18 and lower 19 parts into communication.

 For example, the valve can be opened during the evacuation, at the beginning of the process, by suction through the pumping orifice 6 of the lower part, then the valve is closed during the introduction of gas into the upper part 18 , which makes it possible to maintain a much higher vacuum in the lower part 19 than in said upper part 18.

 The separation of the enclosure 1 into two parts is intended to ensure better control of the pressure prevailing in each part, and consequently, to create a sufficient vacuum to avoid the risks of initiation which may exist at the level of the electron gun in excessively large amount of gas.

 We will now briefly summarize the principle of ionic deposition at low pressure. After making a high vacuum in the enclosure, a gas is introduced under a low pressure, of the order of 10 2 Pa, and a discharge is carried out by application of a negative voltage (a few thousand volts) relative to the mass of the enclosure to the substrate-carrying cathode. The positive ions of the plasma thus created are then accelerated and projected onto the cathode. In the case of the present invention, these ions come either from titanium vaporized by the electron gun, or from gas- introduced into the enclosure and whose ionization is carried out by the electrons released by a hot cathode (here constituted by filament 14).

 The ionic deposit on the substrate is formed by a layer of material corresponding to the projected ions. It is easily understood that the "hooking" on the substrate depends on the energy of the projected ions and therefore on the cathode voltage, and on the gas pressure.

If the voltage increases and the pressure decreases, the energy of the ions increases and their fixation on the substrate is more tenacious.

However, if the energy becomes too great, there is a pickling effect by ions which do not bind. This effect is used, as will be seen below, to clean this substrate, before deposition.

 The function of the filament 14 is to emit electrons to ionize the gas introduced into the upper part 18 of the enclosure 1. The electrons emitted by the filament pass through the zone of plasma created around the hot cathode (filament 14) several times before reaching the anode grid constituted by the third electrode 9, which is positively polarized with respect to the mass of the enclosure.

 In accordance with the invention, the third electrode 9, shown in FIG. 2, consists of a tube 20, part of which is bent in a substantially circular or elliptical shape, and supports a grid 21. More precisely, in a mode particular embodiment, the third electrode is similar to a racket whose frame is constituted by the tube 20, and on which is fixed, like a sieve, the mesh 21. The two ends of the tube 20 are placed parallel to one 'other and angled so that the mesh is parallel to the underside of the substrate-carrying cathode 8, is perpendicular to the vertical axis of the device, and held, by the vertical ends of the tube 20, at their passage tight through the cover 17. A particular feature of the third electrode 9 is that it is cooled by circulation of water in the tube 20.

 The function of this third electrode 9 is to capture the electrons circulating in the plasma and to avoid their dispersion on the walls of the enclosure, which could occur to the detriment of the concentration of the plasma. It is recalled that the concentration of the plasma is also favored by the magnetic field produced by the solenoids 12, 13, and that the screen 15 participates in the non-dispersion of the ions attracted by the cathode 8.

The mobile screen 16 prevents, on the one hand, the passage of ions
Titanium towards cathode 8, when one wants to stop the projection of Titanium, and on the other hand, the pollution of titanium by particles coming from plasma, when the electronic bombardment of titanium is interrupted.

 We will now describe a typical deposition cycle of the hard composite layers in accordance with the process which is the subject of the invention.

 After placing the substrate on the substrate holder 8, and placing the titanium in its evaporation device, the enclosure 1 is hermetically closed, and a high initial vacuum is created there (about 6.5 × 10 5 Pa) . an inert gas, preferably argon, is introduced under a pressure of approximately 0.65 Pa. A discharge is then carried out in the argon at a voltage of 3000 V, that is to say that the substrate holder cathode 8 is polarized at -3,000 V relative to the mass of the enclosure, and with an intensity of 50 mA. This has the effect of bombarding the surface of the substrate with Ar ions having an energy of 1 to 3 KeV, and thus of cleaning it to prepare its surface for deposition.

 The titanium contained in the evaporation device 3 is vaporized by electron bombardment emitted by the electron gun 5, and the movable screen 16 being retracted, a deposit of Ti is formed on the substrate. This deposit can be more or less accentuated by the simultaneous bombardment of Ar + ions. Likewise, the thickness of the layer of pure titanium is adjusted, by varying the duration of the deposition.

 By maintaining the vaporization of the titanium, the argon is gradually replaced by nitrogen, and the deposition is continued, which is then a deposition of TiN, under a nitrogen pressure of 6.5.1O2 at 0.13 Pa, and a discharge voltage of 2000 V with a discharge current of 250 mA. During the deposition, the substrate-carrying cathode is cooled to maintain the temperature of the substrate at 3000 C. Due to the gradual variation of the parameters, in particular the pressures of the different gases, there is no clear interface between two overlapping layers, but a gradual variation in composition, which is an important factor in the adhesion of the different layers.

 Then, the nitrogen pressure is gradually reduced and simultaneously the device is supplied with benzene by gradually increasing its pressure, while keeping the evaporation of the titanium constant. This progresses gradually from the layer of titanium nitride to that of carbide, by creating a layer of carbonitride.

 The evaporation of titanium is then stopped, simultaneously with the closing of the nitrogen supply, the device continuing to be supplied with benzene at a pressure of 6.5 × 10 -2 Pa. The i-carbon is then deposited under a voltage of 1000 V with a discharge current of 40 mA. The substrate temperature is kept at a maximum of 2000 C.

The cycle ends with the cessation of the discharge, the supply of benzene, the return to atmospheric pressure and the removal of the substrate.
As can be seen in the description which has just been made, the transition layer of titanium carbonitride has a concentration of nitrogen decreasing regularly, and a concentration of carbon increasing from the layer of titanium nitride, towards the layer of carbon.

 It is also possible to omit the step of depositing titanium carbonitride, by rapidly switching from the supply of nitrogen to that of benzene. In this case, the intermediate layer of Ti (C, N) is formed directly by implanting carbon ions or hydrogenated carbon in the previously deposited layer of TiN. The energy of the ions is 0.8 to 3 KeV, and an atomic mixture is formed ensuring the connection of the neighboring layers.

 The multilayer coating produced by ionic deposition has high hardness properties. low coefficient of friction, and resistance to corrosion, its application in many fields, from mechanics to medicine as described above, and also as a decorative layer. The layers can be produced on any substrate such as steel, hard alloys, ceramics, glass, etc.

 Typically, the thickness of the layers is between 1 and 20> μm, depending on the deposition parameters. Of course, other thicknesses can be obtained, while remaining within the scope of the invention. Likewise, the device and the cycle described above by way of example are in no way limiting and the values of the pressures, temperature, voltage and intensity are given by way of example. Various modifications may be made by those skilled in the art to the methods and devices described above without departing from the scope of the invention defined by the claims.

Claims (11)

 1. Hard multilayer coating produced by ionic deposition on the surface of a substrate, comprising a layer of titanium nitride, characterized in that it further comprises, on the layer of titanium nitride, firstly a layer of transition from titanium carbonitride, then a top coat of i-carbon.  2. Hard coating according to claim 1, characterized in that the transition layer of titanium carbonitride has a concentration of nitrogen decreasing steadily and a concentration of carbon increasing, starting from the layer of titanium nitride towards the layer of i- carbon.  3. Hard coating according to one of claims 1 and 2, characterized in that it further comprises a layer of titanium deposited between the substrate and the layer of titanium nitride.  4. A process for the preparation of a multilayer hard coating by ionic deposition comprising the deposition, on the surface of a substrate, of a layer of titanium nitride, characterized in that the layer of titanium nitride, the ionic deposition of a layer of titanium carbonitride, then of a layer of i-carbon.  5. Method according to claim 4, characterized in that, prior to the deposition of the titanium nitride layer, an ionic deposition of a layer of pure titanium is carried out directly on the surface of the substrate.  6. Method according to one of claims 4 and 5, characterized in that one successively performs  - cleaning of the substrate by ion bombardment obtained by discharge in an inert gas1  - deposition of the titanium layer by evaporation of the titanium while maintaining the discharge,  - the deposition of the titanium nitride layer by gradual replacement of the inert gas with nitrogen,  - the deposition of the titanium carbonitride transition layer by gradual replacement of nitrogen with a hydrocarbon,  - The deposition of the i-carbon layer by stopping the evaporation of titanium and removing nitrogen, and continuing the discharge in the hydrocarbon.  7. Method according to claim 6, characterized in that at the end of the titanium nitride deposition, the nitrogen is quickly replaced by a hydrocarbon, so that the transition layer of titanium carbonitride is formed directly by implanting carbon ions or hydrogenated carbon emanating from the hydrocarbon, on the layer of titanium nitride.  8. Method according to one of claims 6 and 7, characterized in that the inert gas is argon and that the hydrocarbon is benzene.  9. Method according to one of claims 4 to 8, characterized in that the temperature of the substrate is maintained at approximately 3000 C during the deposition of titanium nitride and at approximately 2000 C during the deposition of i-carbon.  10. Ion deposition device for implementing the method according to one of claims 4 to 8, comprising a sealed enclosure (1) comprising a cover (17), and in which are placed: a substrate-carrying cathode (3 ), surrounded by a screen (15), a titanium evaporation device (3) comprising an electron gun (5), gas supply means (10, 11), a retractable movable screen (16) located above the titanium evaporation device (3), and the orifices (6, 7) connected to a vacuum pump, characterized in that the enclosure (1) consists of two chambers (18, 19) superimposed separated by an intermediate partition (2), each chamber has an orifice (6, 7), connected to a vacuum pump, the electron gun (5) is arranged at the level of the intermediate plate (2), the upper chamber (18) is provided on its periphery with solenoids (12, 13), a tungsten filament (14) heated by Joule effect is placed between the substrate-carrying cathode (8) and the screen m obile (16), and a third positively polarized electrode (9) is placed above said filament (14) and under said cathode (8).  11. Device according to claim 10, characterized in that the third electrode (9) consists of a tube (20) of which a part is curved and supports a grid (21) arranged parallel to the underside of the cathode holder substrate and that said third electrode is cooled by circulation of water in said tube.